Add "Mark all as read" button to NotificationCenter

[Kingsman-99]

Description

NotificationCenter allows users to mark individual notifications as read, but with many unread notifications this is tedious. A "Mark all as read" button at the top of the list clears all unread badges in a single action.

Acceptance Criteria

• A "Mark all as read" button is visible when there is at least one unread notification
• Clicking it marks all currently listed notifications as read immediately (optimistic update)
• The button is hidden when there are no unread notifications
• The action is persisted via the notifications API
• Existing tests continue to pass

Context

• Target file: src/components/NotificationCenter.tsx